Why chimneys get rendered
Render is sometimes applied to chimneys with softer or damaged brickwork underneath, or simply to match a rendered property elsewhere — either way, it needs to stay intact to do its job of shedding water.
Signs render has failed
Cracking, bulging, or render that sounds hollow when tapped usually means water has got in behind it, which can actually trap moisture against the brickwork rather than protecting it.
Repair vs full renewal
Small cracks can sometimes be patched, but render that's failed extensively is usually better stripped back and renewed properly rather than patched repeatedly over a masonry surface that may already be compromised underneath.